With the growing number of therapies being tested in chronic lymphocytic leukemia (CLL) comes the growing challenge of identifying for each patient which therapies are the best. In this competitive renewal proposal, we propose strategies to identify therapeutic opportunities in individual CLL tumors. A key element of this approach is the ability to predict patient response to individual therapies. We apply many of the basic lessons we have learned in the initial grant period about control of apoptosis, and make use of a tool that we refined in that proposal, BH3 profiling. Building on our prior work in which we found we can correlate in vitro sensitivity to ABT-737 to BH3 profiling results, we propose to test our ability to predict clinical response to the related BCL-2 antagonist, ABT-199 in CLL (Specific Aim #1). In addition, in Specific Aim #2, we propose to identify pathway addiction in individual cases of CLL. In the past, inefficiency of siRNA and shRNA approaches and difficulty of ex vivo culture in CLL have made this issue difficult to address by conventional means. We propose a study using small molecule pathway inhibitors instead of knockdown strategies to evade RNA transfection difficulties. Importantly, we use BH3 profiling to measure early apoptotic signaling within 4-24 of drug treatment, obviating the need for extended ex vivo culture. Our aim is to link early apoptotic signaling in response to small molecule inhibitors to pathway addiction. An important advantage of this approach is that detection of apoptotic signaling in primary CLL in response to drugs provides a rational path to clinical translation. Finally, in Specific Aim #3, we propose to investigate how stromal interactions inhibit apoptotic signaling in CLL cells. Moreover, using in vitro co-culture systems, we will study the efficacy and mechanisms of drugs intended to interrupt CLL interactions with stromal cells. Our goal is to identify which CLL patients will most benefit from such therapies.